A brewing device includes a brewing cylinder and a piston received in and reciprocally movable inside the brewing cylinder. The brewing device is connected to a water supply device that supplies a liquid to the brewing cylinder. The piston includes a piston head that is hollow and includes a bar mounted to an inside wall thereof and a piston rod that forms a pivot hole rotatably jointed to the bar to allow the bar to do movement. The piston rod forms a lid at a location close to the pivot hole that selectively sets the piston in a closed condition, whereby through change of position of the lid by the piston rod, the piston is switchably set in an open configuration and a closed configuration for readily controlling amount of water contained in the brewing cylinder to meet various conditions for different quantities of brewing water.